flooding

Flood-risk planners await next-generation laser mapping

Big storms have a way of exposing poor planning and outdated technologies. The Federal Emergency Management Agency took a lot of heat for its response to Hurricane Katrina in 2005. And when Hurricane Sandy struck major parts of New York and New Jersey in October 2012, many buildings that were not in flood-prone areas – at least according to FEMA's Flood Risk Information Maps – were swamped.

FEMA shouldn’t be blamed too quickly for the inaccuracy of its flood-risk maps, however. While there have recently been revolutionary improvements in the accuracy of flood maps and in the predictive power of hydrologic modeling, by 2012 only a fraction of the country had been mapped using the newer technologies.

"The biggest change in the accuracy of flood zones and flood hazard maps in general is due to LiDAR," said Marie Peppler, flood and hazards program coordinator at the U.S. Geological Survey. 

LiDAR (light detection and ranging) devices emit laser pulses and measure the time required for the pulse to reflect back to the device. According to Mark DeMulder, chief of USGS's National Geospatial Program, current LiDAR equipment deployed on aircraft by USGS contractors can measure the elevation of the underlying terrain within 9.25 centimeters.

Prior to LiDAR – from the early 1900s to the early 21st century – most elevation maps were created using manual surveying methods. The old contour maps have a margin of error in elevation accuracy as high as plus or minus 20 feet.  "So we're going from plus or minus 20 feet to 9.25 centimeters," DeMulder said.

And when it comes determining flood risk, elevation measurements are the most critical factor, according to Peppler.  "The accuracy of the map is 110 percent dependent on the accuracy of your elevation data," she said.  "As we get better elevation data sets we can do better modeling on top of that."

Elevation mapping, of course, is useful for many other purposes besides assessing flood risk.  Among other things, LiDAR data can be used to monitor the health of crops, to detect archeological artifacts, to measure the biomass of forests and to perform mineral exploration.    

As a result, a group of federal agencies – including USGS, the National Oceanic and Atmospheric Administration and NASA – work in close concert to manage collection of LiDAR data.

In fact, most of the actual collection of LiDAR data is performed by contractors. "We contract with the private sector for acquisition of the data," DeMulder said. "One of the advantages of operating that way is that the capital investment in equipment and refresh of technology happens in the private sector, where it is driven by competition." 

The resulting data sets are available to other agencies, and even to the general public. "Our data is in the public domain," DeMulder said. "Our philosophy is that if the federal government is producing it, the taxpayers already paid for it. So access to the data is available without additional charge and without a copyright or any kind of use restriction."

3D Elevation Program

While individual agencies may undertake, or contract out a LiDAR survey for a specific purpose, such as to assess storm damage, the USGS is coordinating a program to collect high-resolution elevation data for the entire United States.  Known as 3DEP – the 3D Elevation Program – the effort is expected to take eight years. 

The LiDAR being used for 3DEP is not only highly accurate for measuring elevation, but it also offers greater resolution than earlier technologies, including previous versions of LiDAR.

"Our specification requires two data points for every square meter of terrain," said Larry Sugarbaker, a senior adviser at the USGS National Geospatial Program, which manages 3DEP.  "Historically, it's been about 0.7 points per square meter. So it's a little over twice as dense as what we have collected historically. We're going from measuring accuracy in meters to measuring accuracy in centimeters."

Sugarbaker credits a number of factors for making LiDAR more accurate and less costly. Because the instruments are being improved, the scan rates are increasing and the optics are improving, contractors are collecting more data for a lower cost. Moreover, the processing cost for LiDAR is going down.

According to DeMulder, the resulting data set will provide much more than terrain elevation data.  "It also includes information about all of the features that sit on top of the terrain, such as buildings, power lines, roads – any feature that would return a laser beam from the sensor on board the aircraft," he said.

About the Author

Patrick Marshall is a freelance technology writer for GCN.

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